1. Field of the Invention
The present invention relates to position detectors, and more particularly to a position detector suitable for use in conjunction with a positron CT (computed tomography) for detecting a scintillator's position on which a radiant ray is incident.
2. Description of Prior Art
A conventional position detector detects the scintillator's position on which a radiant ray is incident by way of a current to resistance relation. Specifically, with a photomultiplier tube having a plurality of anodes connected to one another with a resistor interposed between adjacent anodes, currents derived from the two end terminals are measured to determine the radiant ray incident position on the photocathode of the multiplier tube.
Japanese Patent Laid-Open Publication (hereinafter referred to as "OPI") No. 63-300986 discloses another type of a position detector for detecting a position of a light or radiant ray without carrying out computations. OPI No. 60-214287 discloses still another type of a position detector for used in a positron CT.
To improve the position detecting resolution, the position detection pursuant to the current to resistance relation requires an integration circuit for integrating the output signals obtained from the anodes and a processing circuit for computing the light incident position. This technique poses problems in accuracy of the number of photoelectrons counted and also in the complicatedness of the circuits required. Further problem exists such that the discretion of the illuminating points at the end portions of the photocathode are difficult in comparison with that of the illuminating points at the center of the photocathode.
The device disclosed in OPI No. 63-300986 is accurate in counting the number of photoelectrons and has a quick position detecting capability. However, the position detecting resolution is determined depending upon the number of anodes, since the light illuminating segments, e.g. discrete scintillation crystals, and the anodes need to be provided in one-to-one correspondence. If it is contemplated to improve the position detecting resolution with the use of a thin scintillator crystals, the number of anodes increase correspondingly and further the number of signal processing circuits increases a great deal.
The detector disclosed in OPI No. 60-214287 also poses a problem such that to improve the resolution, thin photomultiplier tubes corresponding to thin scintillation crystals are required.